1. Field of the Invention
The present invention relates to a device for cooling a heat source of a motor vehicle and to a refrigerant circulation system for a motor vehicle.
2. Description of the Background Art
Powerful energy accumulators such as lithium ion and NiHM batteries or “super caps” are used in modern hybrid electric vehicles or electric vehicles (HEV/EV vehicles). Resistances within and outside the cells cause these energy accumulators to heat up during rapid charging and discharging. Temperatures above 50° C. permanently damage the energy accumulators. To ensure the operation of the energy accumulators, the latter must be actively cooled. For this purpose, the energy accumulators are brought into contact with a plate through which refrigerant flows and are cooled thereby. The battery cooling plate is integrated into the existing air conditioning refrigeration circuit within the vehicle as an additional, second evaporator (dual evaporator system).
When cooling the cells, it is important for all cells to be cooled uniformly. As a result, a maximum temperature gradient of around 5K must be maintained in the cooling plate. If cell cooling is not uniform, the cells age at different rates, which has a negative impact on the operation and performance of the energy accumulator.
FIG. 4 shows a refrigerant circulation system according to the conventional art, in which a battery cooling plate 402 is series-connected to an air conditioner evaporator 404. In addition to battery cooling plate 402 and air conditioner evaporator 404, the refrigerant circulation system has a compressor 406, a condenser 408, a valve 412 having a fixed opening (FXV) and a thermal expansion valve (TXV) 414.
Cooling plate 402 for cooling the energy accumulator is always operated in “wet” mode to ensure uniform cooling of all energy accumulator cells. This means that battery cooling plate 402 must always be provided with enough refrigerant that the physical processes within battery cooling plate 402 always take place in the wet steam region. To ensure uniform temperature distribution on plate 402, overheating should never occur in plate 402.
Transferred to the refrigeration circuit in the vehicle, this means that it is not possible to introduce battery cooling plate 402 directly upstream from compressor 408 in a manner analogous to the interconnection of air conditioner evaporator 404. After all, refrigerant is still present in the outlet of plate 402, so that compressor 406 may be exposed to liquid impact, which would cause damage.
Battery cooling plate 402 is therefore series-connected to main air conditioner evaporator 404. The still present fluid portion of the refrigerant is re-evaporated in main air conditioner evaporator 404, thereby preventing liquid impact at compressor 406. The mass flow through battery cooling plate 404 is determined by valve 412 having a fixed opening. The fixed opening diameter of valve 412 is determined by the maximum load in the battery.
FIG. 5 shows a battery cooling plate 520 according to the conventional art. Battery cooling plate 520 has a plurality of refrigerant passages 526. Refrigerant passages 526 have a common inlet 532 and a common outlet 534.
The operating points for battery cooling plate 520 are set in the circuit in such a way that the operating point of plate 520 is always in the wet steam region, and the refrigerant in plate 520 thus has approximately the same temperature. In designing the flow passages in plate 520, it is necessary to ensure, in particular, that the refrigerant is distributed uniformly to passages 526, that the pressure loss in plate 520 is not excessive, and that sufficient passages 526 are provided for uniform cooling of plate 520. To ensure uniform distribution of the refrigerant, one option is to allow the refrigerant to enter at a point 532 on plate 520 so that the refrigerant may be distributed to individual passages 526 in plate 520. Passages 526 converge again at outlet point 534. Feed and return flows are situated at a relatively great spatial distance from each other.
The interconnection of the battery cooling plate shown in FIG. 4 in the vehicle refrigerant circuit has the disadvantage that the battery cooling system is always coupled with the air conditioning system of the vehicle compartment. It is not possible to operate the battery cooling plate alone without the air conditioner evaporator. This has enormous disadvantages, particularly in winter, since the air conditioner evaporator is operated only up to specific outdoor temperatures to prevent icing. Specifically, this means that the battery may no longer be operated at cold outdoor temperatures.